Lithium ion secondary battery and method for manufacturing the same

ABSTRACT

A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte provided between the positive electrode and the negative electrode. The positive electrode includes a positive electrode current collector and a positive electrode active material layer over the positive electrode current collector. The positive electrode active material layer includes a plurality of lithium-containing composite oxides each of which is expressed by LiMPO 4  (M is one or more of Fe (II), Mn (II), Co (II), and Ni (II)) that is a general formula. The lithium-containing composite oxide is a flat single crystal particle in which the length in the b-axis direction is shorter than each of the lengths in the a-axis direction and the c-axis direction. The lithium-containing composite oxide is provided over the positive electrode current collector so that the b-axis of the single crystal particle intersects with the surface of the positive electrode current collector.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a lithium ion secondary battery and amethod for manufacturing the lithium ion secondary battery.

2. Description of the Related Art

In recent years, lithium ion secondary batteries have been underdevelopment. Because of their high thermal stability, lithium-containingcomposite oxides having olivine structures, such as LiFePO₄, LiMnPO₄,LiCoPO₄, and LiNiPO₄, have been expected as positive electrode activematerials of lithium ion secondary batteries. Such a lithium-containingcomposite oxide having an olivine structure contains a bivalenttransition metal element (e.g., Fe, Mn, Co, and Ni).

As a method for manufacturing lithium-containing composite oxides havingolivine structures, a solid phase method, a hydrothermal method, asol-gel method, or the like is used (e.g., Patent Document 1). In orderto increase the discharge capacity and the energy density of a lithiumion secondary battery, attempts have been made to reduce the particlediameters and variation in particle size of an active material includedin an active material layer that relates to insertion and extraction ofions functioning as carriers. A hydrothermal method has been used as amethod for manufacturing lithium-containing composite oxides with lessvariation in particle size and small particle diameters.

REFERENCE Patent Document

-   [Patent Document 1] PCT International Publication No. 08/077447    pamphlet

SUMMARY OF THE INVENTION

However, lithium-containing composite oxides included in a lithium ionsecondary battery have high resistance, so that there has been a limiton the increase of the discharge capacity and the energy density.

In view of the above, an object of one embodiment of the presentinvention is to provide a lithium ion secondary battery having higherdischarge capacity and higher energy density and a method formanufacturing such a lithium ion secondary battery.

One embodiment of the present invention is a lithium ion secondarybattery including a positive electrode, a negative electrode, and anelectrolyte provided between the positive electrode and the negativeelectrode. The positive electrode includes a positive electrode currentcollector and a positive electrode active material layer provided overthe positive electrode current collector. The positive electrode activematerial layer includes a plurality of lithium-containing compositeoxides. The lithium-containing composite oxide is expressed by LiMPO₄ (Mis one or more of Fe (II), Mn (II), Co (II), and Ni (II)) that is ageneral formula. The lithium-containing composite oxide is a flat singlecrystal particle. In the flat single crystal particle, the length in theb-axis direction is shorter than each of the lengths in the a-axisdirection and the c-axis direction. The length in the b-axis directionis typically greater than or equal to 5 nm and less than or equal to 50nm. The b-axis of the single crystal particle of the lithium-containingcomposite oxide forms a given angle with a surface of the positiveelectrode current collector, and the b-plane of the single crystalparticle is in contact with the positive electrode current collector. Inother words, the lithium-containing composite oxide is provided over thepositive electrode current collector so that the b-axis of the singlecrystal particle intersects with the surface of the positive electrodecurrent collector. Typically, the b-axis of the single crystal particleintersects with the surface of the positive electrode current collectorat an angle greater than or equal to 60 degrees and less than or equalto 90 degrees.

Note that the lithium-containing composite oxide has an olivinestructure. The lithium-containing composite oxide has an orthorhombiccrystal structure and belongs to a space group Pnma (62). In the singlecrystal particle of the lithium-containing composite oxide, the lengthsin the a-axis direction and the c-axis direction are each longer thanthe length in the b-axis direction. In the positive electrode activematerial layer, the lithium-containing composite oxides may be stackedon each other.

Further, one embodiment of the present invention is a method formanufacturing a lithium ion secondary battery, which includes the stepsof applying slurry including a lithium-containing composite oxide thatis a flat single crystal particle whose length in the b-axis directionis greater than or equal to 5 nm and less than or equal to 50 nm to apositive electrode current collector; and exerting pressure on ortransmitting vibration to the slurry including the lithium-containingcomposite oxide so that the lithium-containing composite oxide isprovided over the positive electrode current collector while the b-axisof the lithium-containing composite oxide forms a given angle with asurface of the positive electrode current collector and the b-plane isin contact with the positive electrode current collector. In otherwords, the lithium-containing composite oxide is provided over thepositive electrode current collector so that the b-axis of the singlecrystal particle intersects with the surface of the positive electrodecurrent collector.

In the positive electrode of the lithium ion secondary battery that isone embodiment of the present invention, the positive electrode activematerial layer includes an olivine lithium-containing composite oxidethat is a flat single crystal particle in which the length in the b-axisdirection is shorter than each of the lengths in the a-axis directionand the c-axis direction. Further, a plane including the a-axis and thec-axis (b-plane) is in contact with the positive electrode currentcollector and the b-axis forms a given angle with the positive electrodecurrent collector at a given angle. In other words, the b-axisintersects with a surface of the positive electrode current collector.Therefore, migration of lithium ions between the current collector andan electrolyte is easy. With the use of the positive electrode activematerial layer having this structure for a positive electrode, a lithiumion secondary battery with reduced internal resistance and higher powercan be provided.

According to one embodiment of the present invention, the dischargecapacity of a lithium ion secondary battery can be increased, and thelithium ion secondary battery can have higher power. Further, a lithiumion secondary battery with higher discharge capacity and higher powercan be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are perspective views showing positive electrodes of alithium ion secondary batteries.

FIG. 2 shows a crystal structure of olivine LiFePO₄.

FIGS. 3A to 3C are perspective views showing a method for forming apositive electrode of a lithium ion secondary battery.

FIG. 4 is a cross-sectional view showing a lithium ion secondarybattery.

FIGS. 5A and 5B are perspective views showing an application of alithium ion secondary battery.

FIG. 6 shows an example of a structure of a wireless power feedingsystem.

FIG. 7 shows an example of a structure of a wireless power feedingsystem.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described withreference to the drawings below. Note that the present invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be interpreted asbeing limited to the following description of the embodiments. Indescription with reference to the drawings, in some cases, the samereference numerals are used in common for the same portions in differentdrawings. Further, in some cases, the same hatching patterns are used insimilar parts, and the similar parts are not necessarily designated byreference numerals.

Embodiment 1

In this embodiment, a positive electrode of a lithium ion secondarybattery that is one embodiment of the present invention and a method forforming the positive electrode of the lithium-ion secondary battery willbe described with reference to FIGS. 1A to 1C, FIG. 2, and FIGS. 3A to3C.

FIGS. 1A to 1C are perspective views of positive electrodes of lithiumion secondary batteries.

As shown in FIG. 1A, lithium-containing composite oxides 103 which arepositive electrode active materials are provided over a positiveelectrode current collector 101.

In FIG. 1B, lithium-containing composite oxides 103 a and 103 b, whichare positive electrode active materials, are stacked over the positiveelectrode current collector 101.

Note that a region over the positive electrode current collector 101,which includes a plurality of positive electrode active materials, iscalled a positive electrode active material layer. Although not shown,the positive electrode active material layer may include a conductionauxiliary agent, a binder, and the like. The positive electrode activematerial refers to a substance that relates to insertion and extractionof ions functioning as carriers. Thus, the lithium-containing compositeoxides are positive electrode active materials but a carbon layer, aconduction auxiliary agent, a binder, and a solvent are not activematerials.

A material having high conductivity such as platinum, aluminum, copper,titanium, or stainless steel can be used for the positive electrodecurrent collector 101. The positive electrode current collector 101 canhave a shape such as a foil shape, a plate shape, or a net shape asappropriate.

As the thickness of the positive electrode active material layer, adesired thickness is selected from the range of 20 μm to 100 μm. It ispreferable to adjust the thickness of the positive electrode activematerial layer as appropriate so that cracks and separation do notoccur.

The lithium-containing composite oxide included in the positiveelectrode active material layer is a single crystal particle having anolivine structure. Typical examples of the olivine lithium-containingcomposite oxide (general formula thereof is LiMPO₄ (M is one or more ofFe (II), Mn (II), Co (II), and Ni (II))) include LiFePO₄, LiNiPO₄,LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄,LiMn_(a)Co_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)CO_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1,and 0<e<1), LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, <f<1, 0<g<1,0<h<1, and 0<i<1), and the like.

Note that a surface of the lithium-containing composite oxide may becovered with a carbon layer with a thickness less than or equal to 10nm, preferably greater than or equal to 1 nm and less than or equal to10 nm.

Here, the shape of the lithium-containing composite oxide that is apositive electrode active material will be described with reference toFIG. 1C.

The lithium-containing composite oxide 103 has an orthorhombic crystalstructure and belongs to a space group Pnma (62). The lithium-containingcomposite oxide 103 is a flat single crystal particle in which thelength in the b-axis direction is shorter than each of the lengths inthe a-axis direction and the c-axis direction. Since lithium ions arediffused in the b-axis direction in an olivine structure, it ispreferable to set the length in the b-axis direction to longer than orequal to 5 nm and shorter than or equal to 50 nm, preferably longer thanor equal to 5 nm and shorter than or equal to 20 nm so that lithium ionsare easily diffused. Further, it is preferable to set the ratio of thelengths in the a-axis direction and the c-axis direction to greater thanor equal to 0.5 and less than or equal to 1.5, preferably greater thanor equal to 0.8 and less than or equal to 1.2, i.e., the b-plane havinga square shape or substantially square shape is preferable, because thelithium-containing composite oxides 103 can be arranged densely over thepositive electrode current collector 101.

In addition, in the lithium-containing composite oxide, a planeincluding the a-axis and the c-axis, i.e., the b-plane is in contactwith the positive electrode current collector 101, and the b-axis formsa given angle with a surface of the positive electrode current collector101 at a given angle. In other words, the b-axis of the single crystalparticle intersects with the surface of the positive electrode currentcollector 101. The b-axis of the lithium-containing composite oxideintersects with the surface of the positive electrode current collector101 at an angle typically greater than or equal to 60 degrees and lessthan or equal to 90 degrees. Since lithium ions are diffused in theb-axis direction in an olivine structure, it is preferable that theb-axis intersects with the surface of the positive electrode currentcollector 101 at an angle greater than or equal to 60 degrees and lessthan or equal to 90 degrees to diffuse a larger number of lithium ions.Note that the term “the b-axis intersects with the surface of thepositive electrode current collector 101” means that the b-axis and thesurface of the positive electrode current collector 101 have anintersection point. In contrast, the term “the b-axis does not intersectwith the surface of the positive electrode current collector 101” meansthat the b-axis is in parallel to the surface of the positive electrodecurrent collector 101.

Note that it can be judged that the lithium-containing composite oxide103 is a flat single crystal particle in which the length in the b-axisdirection is shorter than each of the lengths in the a-axis directionand the c-axis direction, by using more than one of a scanning electronmicroscope (SEM), a scanning transmission electron microscope (STEM), atransmission electron microscope (TEM), and X-ray diffraction (XRD). Forexample, X-ray diffraction (XRD) measurement can show that the b-axis ofthe single crystal particle of the lithium-containing composite oxide103 intersects with the surface of the positive electrode currentcollector 101. Further, the lithium-containing composite oxide 103 isjudged as a single crystal particle because the contrast of a dark-fieldimage observed with a transmission electron microscope (TEM) is uniformand thus grain boundaries are not seen in the dark-field image.

Here, description is given of an olivine structure. FIG. 2 shows a unitcell 301 of lithium iron phosphate (LiFePO₄) that is an example of anolivine lithium-containing composite oxide. An olivine lithium ironphosphate has an orthorhombic crystal structure, and includes fourformula units of lithium iron phosphate (LiFePO₄) within a unit cell.The basic framework of the olivine structure is a hexagonal close-packedstructure of an oxide ion, in which lithium, iron, and phosphorus arelocated in the close-packed gaps.

Further, the olivine lithium iron phosphate (LiFePO₄) has a tetrahedralsite and two kinds of octahedral sites. The tetrahedral site has fouroxygen atoms in the vertices. The octahedral sites have six oxygen atomsin the vertices. Phosphorus 307 is located at the center of thetetrahedral site, and lithium 303 or iron 305 is located at the centerof the octahedral sites. The octahedral site with the lithium 303located at the center is called a M1 site, and the octahedral site withthe iron 305 located at the center is called a M2 site. The M1 site isarranged one-dimensionally in the b-axis direction. In other words, thelithium 303 is arranged one-dimensionally in a <010> direction. Notethat for convenience, bonds between the lithium 303 and other ions oratoms are not shown by lines.

The irons 305 of neighboring M2 sites are bonded in a zigzag manner withoxygen 309 provided therebetween. Also, the oxygen 309 bonded betweenthe irons 305 of the neighboring M2 sites, is also bonded to thephosphorus 307 of the tetrahedral site. Thus, the bond between the ironatom and the oxygen atom and the bond between the oxygen atom and thephosphorus atom are continuous.

Note that the olivine lithium iron phosphate may be distorted.Furthermore, regarding the lithium iron phosphate, the composition ratioof the lithium, the iron, the phosphorus, and the oxygen is not limitedto 1:1:1:4. Also, as the transition metal (M) of a lithium transitionmetal phosphate (LiMPO₄), a transition metal which has a larger ionicradius than a lithium ion, such as manganese, cobalt, or nickel, may beused.

Even iron phosphate alone from the olivine lithium iron phosphate shownin FIG. 2 has a stable structure. For this reason, insertion andextraction of all lithium ions are possible. Further, the olivinelithium iron phosphate has heat stability. In the b-axis direction inthe olivine lithium iron phosphate, lithium ions are arrangedone-dimensionally and are diffused in the b-axis direction. Thus,diffusion of lithium ions can be easy in a single crystal particle witha short length in the b-axis direction.

In the positive electrode of this embodiment, the positive electrodeactive material layer includes an olivine lithium-containing compositeoxide which is a flat single crystal particle in which the length in theb-axis direction is shorter than each of the lengths in the a-axisdirection and the c-axis direction. Further, the plane including thea-axis and the c-axis, i.e., the b-plane is in contact with the positiveelectrode current collector, and the b-axis whose direction is thedirection of diffusion of lithium ions in the olivine structureintersects with a surface of the positive electrode current collector.Therefore, the number of diffusion of lithium ions between the currentcollector and an electrolyte can be increased. Furthermore, with the useof the lithium-containing composite oxide of this embodiment for apositive electrode active material in a lithium ion secondary battery,the internal resistance of the lithium ion secondary battery is reduced,so that the lithium ion secondary battery can have higher power anddischarge capacity thereof can be as high as theoretical dischargecapacity. Note that in this specification, an electrolyte means the onethat includes a material in which lithium ions stably exist and withwhich lithium ions functioning as carrier ions can be transferred. Theelectrolyte includes in its category an electrolyte solution obtained bydissolving in a solvent, a material (solute) in which lithium ionsstably exist, and a solid electrolyte including a material (solute) inwhich lithium ions stably exist, for example.

Further, the plurality of lithium-containing composite oxides is stackedin the positive electrode active material layer as shown in FIG. 1B,whereby the lithium ion secondary battery can have discharge capacityhigher than that in the case of FIG. 1A.

Next, a method for forming the positive electrode of the lithium ionsecondary battery shown in FIGS. 1A to 1C will be described withreference to FIGS. 3A to 3C.

Slurry 105 including the lithium-containing composite oxides 103 isapplied to the positive electrode current collector 101. Then, it ispreferable to make the thickness of the slurry 105 including thelithium-containing composite oxides 103 approximately uniform with theuse of a squeegee, a blade, or the like. Further, a solvent of theslurry 105 may be dried so that the viscosity of the slurry 105 isincreased. Note that through the steps, as shown in FIG. 3A, thelithium-containing composite oxides 103 are applied randomly to thepositive electrode current collector 101; thus, the a-axis direction,the b-axis direction, or the c-axis direction of each lithium-containingcomposite oxides intersects with the surface of the positive electrodecurrent collector 101. The lithium-containing composite oxide is a flatsingle crystal particle in which the length in the b-axis direction isshorter than each of the lengths in the a-axis direction and the c-axisdirection. Therefore, when the lithium-containing composite oxides 103are dispersed over the positive electrode current collector 101 so thatthe a-axis directions or the c-axis directions thereof intersect withthe surface of the positive electrode current collector 101, i.e., sothat the a-planes or the c-planes thereof are in contact with thepositive electrode current collector 101, the lithium-containingcomposite oxides lie so that the height thereof is high as shown by alithium-containing composite oxide 103 c.

The slurry 105 including the lithium-containing composite oxidesincludes a binder and a conduction auxiliary agent. Note that in FIGS.3A and 3B, the slurry 105 including the lithium-containing compositeoxides is shown by dashed lines.

A solid method, a hydrothermal method, a spray pyrolysis method, or thelike can be used as appropriate for the method for manufacturing thelithium-containing composite oxide. Note that a hydrothermal method ispreferably used for a method for manufacturing flat single crystalparticles which have small particle diameters and less variation inparticle size and in each of which the length in the b-axis direction isshorter than each of the lengths in the a-axis direction and the c-axisdirection.

As the binder, a polysaccharide such as starch, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, or diacetyl cellulose; avinyl polymer such as polyvinylchloride, polyvinyl pyrrolidone,polytetrafluoroethylene, polyvinyliden fluoride, polyethylene,polypropylene, polyvinyl alcohol, ethylene-propylene-diene monomer(EPDM) rubber, sulfonated EPDM rubber, styrene-butadiene rubber,butadiene rubber, or fluorine rubber; polyether such as polyethyleneoxide; or the like is given.

As the conduction auxiliary agent, a material which is itself anelectron conductor and does not cause chemical reaction with othermaterials in a lithium ion secondary battery may be used. For example,carbon-based materials such as graphite, carbon fiber, carbon black,acetylene black, and VGCF (registered trademark); metal materials suchas copper, nickel, aluminum, and silver; and powder, fiber, and the likeof mixtures thereof are given. The conduction auxiliary agent is amaterial that assists conductivity between active materials. Theconduction auxiliary agent is filled between active materials which areapart and makes conduction between the active materials.

Note that a solvent may be used as appropriate to disperse or dissolvethe lithium-containing composite oxides, the binder, and the conductionauxiliary agent in the slurry.

Lithium-containing composite oxides with small particle diameters arelikely to agglomerate and difficult to disperse uniformly in the slurry.For this reason, a dispersant and a disperse medium are preferably usedas appropriate to disperse the lithium-containing composite oxidesuniformly in the slurry.

As the dispersant, a polymer dispersant, a surfactant dispersant(low-molecular dispersant), an inorganic dispersant, and the like aregiven. As the disperse medium, alcohol, water, or the like is given.Note that the dispersant and the disperse medium may be selected asappropriate to suit the lithium-containing composite oxide.

Next, physical pressure is exerted on the slurry 105 including thelithium-containing composite oxide 103. As a method for exertingphysical pressure on the slurry 105 including the lithium-containingcomposite oxides 103, a method of moving a roller, a squeegee, a blade,or the like over the slurry 105 including the lithium-containingcomposite oxides 103 is given. Alternatively, ultrasonic vibration maybe transmitted to the slurry including the lithium-containing compositeoxides, instead of exerting physical pressure on the slurry.Consequently, in the slurry 105 including the lithium-containingcomposite oxides 103, the lithium-containing composite oxide 103 c whosea-axis or c-axis intersects with the surface of the positive electrodecurrent collector 101, i.e., the lithium-containing composite oxide 103c whose a-plane or c-plane is in contact with the positive electrodecurrent collector 101, falls down; thus, the lithium-containingcomposite oxide 103 can be in the state where the b-axis thereofintersects with the surface of the positive electrode current collector101 as shown in FIG. 3B. In addition, the b-plane of thelithium-containing composite oxide 103 can be in contact with thepositive electrode current collector 101. In other words, an area of thepositive electrode current collector 101, which is in contact with thelithium-containing composite oxides 103, can be increased.

Then, the slurry 105 including the lithium-containing composite oxides103 is heated to remove the solvent, and the lithium-containingcomposite oxides 103 are fixed to the positive electrode currentcollector with a binder; thus, a positive electrode active materiallayer 109 is formed (see FIG. 3C). Note that the binder becomes porousand fibrous through the heating and includes gaps, so that thelithium-containing composite oxides are exposed in the gaps. Note thatin FIG. 3C, the positive electrode active material layer 109 is shown bydashed lines.

Through the above steps, the positive electrode of the lithium ionsecondary battery can be formed.

In the positive electrode of this embodiment, the positive electrodeactive material layer includes the olivine lithium-containing compositeoxide which is a flat single crystal particle in which the length in theb-axis direction is shorter than each of the lengths in the a-axisdirection and the c-axis direction. Further, the plane including thea-axis and the c-axis, i.e., the b-plane is in contact with the positiveelectrode current collector, and the b-axis whose direction is thedirection of diffusion of lithium ions intersects with the surface ofthe positive electrode current collector. Therefore, the number ofdiffusion of lithium ions between the current collector and theelectrolyte can be increased. Further, with the use of the positiveelectrode described in this embodiment for a lithium ion secondarybattery, the lithium ion secondary battery can have reduced internalresistance and higher power. Moreover, the lithium ion secondary batterycan have discharge capacity as high as theoretical discharge capacity.

Embodiment 2

In this embodiment, a method for forming a positive electrode activematerial layer including a solid electrolyte over a positive electrodecurrent collector will be described. Note that here, description will bemade with reference to Embodiment 1.

In this embodiment, a binder and a solute of an electrolyte of a lithiumion secondary battery are included in the positive electrode activematerial layer described in Embodiment 1.

As the solute of the electrolyte, a material in which lithium ionsfunctioning as carrier ions can transfer and exist stably is used.Typical examples of the solute of the electrolyte include lithium saltssuch as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N.

Note that a solvent may be used as appropriate to disperse or dissolvethe solute of the electrolyte in slurry.

As shown in FIG. 3A, in a manner similar to that in Embodiment 1, slurry115 including the solute of the electrolyte as well as thelithium-containing composite oxides 103, the binder, and a conductiveauxiliary agent is applied to the positive electrode current collector101. Then, the thickness of the slurry 115 may be made uniform and asolvent of the slurry 115 may be dried. Note that in FIGS. 3A and 3B,the slurry 115 including the lithium-containing composite oxides isshown by dashed lines.

Next, in a manner similar to that in Embodiment 1, physical pressure isexerted on the slurry 115 including the lithium-containing compositeoxides 103. Alternatively, ultrasonic vibration may be transmitted tothe slurry 115 including the lithium-containing composite oxides 103.Consequently, as shown in FIG. 3B, the lithium-containing compositeoxide 103 can be in the state where the b-axis thereof intersects withthe surface of the positive electrode current collector 101. Inaddition, the b-plane of the lithium-containing composite oxide 103 canbe in contact with the positive electrode current collector 101.

Then, in a manner similar to that in Embodiment 1, the slurry 115including the lithium-containing composite oxides 103 and a material ofthe binder are heated to remove the solvent, and the lithium-containingcomposite oxides 103 are fixed to the positive electrode currentcollector with the binder. Thus, a positive electrode active materiallayer filled with a solid electrolyte is formed (see FIG. 3C). Note thatin FIG. 3C, a positive electrode active material layer 119 is shown bythe dashed lines.

According to this embodiment, a positive electrode in which a positiveelectrode active material layer filled with a solid electrolyte isprovided over a positive electrode current collector can be formed;thus, resistance at the interface between the electrode and theelectrolyte can be reduced. As a result, with the use of the positiveelectrode described in this embodiment, the internal resistance of alithium ion secondary battery is further reduced, the lithium ionsecondary battery can have higher power, charge and discharge thereofcan be performed at high speed, and the discharge capacity can be ashigh as theoretical discharge capacity.

Embodiment 3

in this embodiment, a lithium ion secondary battery and a method formanufacturing the lithium ion secondary battery will be described.

The lithium ion secondary battery in this embodiment will be describedwith reference to FIG. 4. Here, description is made below on across-sectional structure of the lithium ion secondary battery.

FIG. 4 is a cross-sectional view of the lithium ion secondary battery.

A lithium ion secondary battery 400 includes a negative electrode 411including a negative electrode current collector 407 and a negativeelectrode active material layer 409, a positive electrode 405 includinga positive electrode current collector 401 and a positive electrodeactive material layer 403, and a separator 413 provided between thenegative electrode 411 and the positive electrode 405. Note that theseparator 413 includes an electrolyte 415. The negative electrodecurrent collector 407 is connected to an external terminal 419 and thepositive electrode current collector 401 is connected to an externalterminal 417. An end portion of the external terminal 419 is embedded ina gasket 421. In other words, the external terminals 417 and 419 areinsulated from each other with the gasket 421.

As the negative electrode current collector 407, a material having highconductivity such as copper, stainless steel, iron, or nickel can beused. The negative electrode current collector 407 can have a shape suchas a foil shape, a plate shape, or a net shape as appropriate.

The negative electrode active material layer 409 is formed using amaterial capable of lithium-ion occlusion and emission. As the negativeelectrode active material layer 409, lithium, aluminum, graphite,silicon, tin, germanium, or the like is typically used. Note that thenegative electrode current collector 407 may be omitted and the negativeelectrode active material layer 409 alone may be used as the negativeelectrode. The theoretical lithium occlusion capacity is larger ingermanium, silicon, lithium, and aluminum than graphite. When theocclusion capacity is large, charge and discharge can be performedsufficiently even in a small area and a function as a negative electrodecan be obtained; therefore, cost can be reduced and a lithium-ionsecondary battery can be miniaturized. However, in the case of siliconor the like, the volume is approximately quadrupled due to lithiumocclusion; therefore, the probability that the material itself getsvulnerable should be considered.

Note that the negative electrode active material layer 409 may bepredoped with lithium. As a predoping method of lithium, a lithium layermay be formed on a surface of the negative electrode active materiallayer 409 by a sputtering method. Alternatively, a lithium foil isprovided on the surface of the negative electrode active material layer409, whereby the negative electrode active material layer 409 can bepredoped with lithium.

A desired thickness of the negative electrode active material layer 409is selected from the range of 20 μm to 100 μm.

Note that the negative electrode active material layer 409 may include abinder and a conduction auxiliary agent. As the binder and theconduction auxiliary agent, the binder and the conduction auxiliaryagent which are included in the positive electrode active material layerdescribed in Embodiment 1 can be used as appropriate.

As the positive electrode current collector 401 and the positiveelectrode active material layer 403, the positive electrode currentcollector 101 and the positive electrode active material layer 109 whichare described in Embodiment 1 can be used as appropriate.

An insulating porous material is used for the separator 413. Typicalexamples of the separator 413 include cellulose (paper), polyethylene,and polypropylene.

As a solute of the electrolyte 415, a material in which lithium ionsfunctioning as carrier ions can transfer and stably exist and which isdescribed in Embodiment 2 is used as appropriate.

As a solvent of the electrolyte 415, a material which can transferlithium ions is used. As the solvent of the electrolyte 415, an aproticorganic solvent is preferably used. Typical examples of aprotic organicsolvents include ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, γ-butyrolactone, acetonitrile,dimethoxyethane, tetrahydrofuran, and the like, and one or more of thesematerials can be used. When a gelled polymer is used as the solvent ofthe electrolyte 415, safety against liquid leakage or the like isincreased. Further, the lithium-ion secondary battery 400 can be madethinner and more lightweight. Typical examples of gelled polymersinclude a silicon gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide, polypropylene oxide, a fluorine-based polymer, andthe like.

As the electrolyte 415, a solid electrolyte such as Li₃PO₄ can be used.Note that in the case of using the solid electrolyte as the electrolyte415, the separator 413 is unnecessary.

Alternatively, as described in Embodiment 2, a positive electrode activematerial layer filled with a solid electrolyte may be provided over thepositive electrode current collector.

For the external terminals 417 and 419, a metal member such as astainless steel plate or an aluminum plate can be used as appropriate.

Note that in this embodiment, a coin-type lithium ion secondary batteryis given as the lithium ion secondary battery 400; however, a lithiumion secondary battery with various shapes, such as a sealing-typelithium ion secondary battery, a cylindrical lithium ion secondarybattery, and a square-type lithium ion secondary battery, can be used.Further, a structure in which a plurality of positive electrodes, aplurality of negative electrodes, and a plurality of separators arestacked or rolled may be employed.

A lithium-ion secondary battery has a high energy density, a largecapacity, and further a high output voltage. For these reasons, the sizeand the weight of the lithium ion battery can be reduced. Further, thelithium ion battery does not easily degrade due to repetitive charge anddischarge and can be used for a long time, so that cost can be reduced.With the use of an olivine lithium-containing composite oxide which is aflat single crystal particle in which the length in the b-axis directionis shorter than each of the lengths in the a-axis direction and thec-axis direction for the positive electrode active material layer, thelithium ion secondary battery can have higher discharge capacity andhigher power.

Next, a method for manufacturing the lithium ion secondary battery 400described in this embodiment will be described.

First, a method for forming the negative electrode 411 will bedescribed.

The negative electrode active material layer 409 is formed over thenegative electrode current collector 407 by a coating method, asputtering method, an evaporation method, or the like, so that thenegative electrode 411 is formed. Alternatively, for the negativeelectrode 411, a foil, a plate, or a mesh of lithium, aluminum,graphite, or silicon can be used. Here, graphite predoped with lithiumis used as the negative electrode.

Next, the method for manufacturing the positive electrode described inEmbodiment 1 is used as appropriate to form the positive electrode 405.

Next, the negative electrode 411, the separator 413, and the positiveelectrode 405 are impregnated with the electrolyte 415. Then, thepositive electrode 405, the separator 413, the gasket 421, the negativeelectrode 411, and the external terminal 419 are stacked in this orderover the external terminal 417, and the external terminal 417 and theexternal terminal 419 are crimped to each other with a “coin cellcrimper”. Thus, the coin-type lithium ion secondary battery can bemanufactured.

Note that a spacer and a washer may be provided between the externalterminal 417 and the positive electrode 405 or between the externalterminal 419 and the negative electrode 411 so that connection betweenthe external terminal 417 and the positive electrode 405 or between theexternal terminal 419 and the negative electrode 411 is enhanced.

Embodiment 4

In this embodiment, an application of the lithium ion secondary batterydescribed in Embodiment 3 will be described with reference to FIGS. 5Aand 5B.

The lithium ion secondary battery described in Embodiment 3 can be usedin electronic devices, e.g., cameras such as digital cameras or videocameras, digital photo frames, mobile phones (also referred to ascellular phones or cellular phone devices), portable game machines,portable information terminals, or audio reproducing devices. Further,the lithium ion secondary battery can be used in electric propulsionvehicles such as electric cars, hybrid cars, railway train vehicles,maintenance vehicles, carts, or electric wheelchairs. Here, an exampleof the electric propulsion vehicle will be described.

FIG. 5A shows a structure of a four-wheeled automobile 500 as an exampleof the electric propulsion vehicles. The automobile 500 is an electricvehicle or a hybrid vehicle. An example is shown in which the automobile500 includes a lithium ion secondary battery 502 provided on its bottomportion. In order to clearly show the position of the lithium ionsecondary battery 502 in the automobile 500, FIG. 5B shows the outlineof the automobile 500 and the lithium ion secondary battery 502 providedon the bottom portion of the automobile 500. The lithium ion secondarybattery described in Embodiment 3 can be used as the lithium ionsecondary battery 502. The lithium ion secondary battery 502 can becharged by a plug-in technique or a wireless power feeding system, whichsupplies power from the outside.

Embodiment 5

In this embodiment, examples of using a lithium ion secondary batteryaccording to one embodiment of the present invention in a wireless powerfeeding system (hereinafter referred to as an RF power feeding system)will be described with reference to block diagrams in FIG. 6 and FIG. 7.In each of the block diagrams, blocks show elements independently, whichare classified according to their functions, within a power receivingdevice and a power feeding device. However, it is practically difficultto completely separate the elements according to their functions; insome cases, one element can involve a plurality of functions.

First, the RF power feeding system will be described with reference toFIG. 6.

A power receiving device 600 is an electronic device or an electricpropulsion vehicle which is driven by electric power supplied from apower feeding device 700, and can be applied to any other devices whichare driven by electric power, as appropriate. Typical examples of theelectronic device include cameras such as digital cameras or videocameras, digital photo frames, mobile phones, portable game machines,portable information terminals, audio reproducing devices, displaydevices, computers, and the like. Typical examples of the electricpropulsion vehicle include electric cars, hybrid cars, railway trainvehicles, maintenance vehicles, carts, electric wheelchairs, and thelike. In addition, the power feeding device 700 has a function ofsupplying electric power to the power receiving device 600.

In FIG. 6, the power receiving device 600 includes a power receivingdevice portion 601 and a power load portion 610. The power receivingdevice portion 601 includes at least a power receiving device antennacircuit 602, a signal processing circuit 603, and a lithium ionsecondary battery 604. The power feeding device 700 includes at least apower feeding device antenna circuit 701 and a signal processing circuit702.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 and transmitting a signal to the power feeding deviceantenna circuit 701. The signal processing circuit 603 processes asignal received by the power receiving device antenna circuit 602 andcontrols charging of the lithium ion secondary battery 604 and supplyingof electric power from the lithium ion secondary battery 604 to thepower load portion 610. In addition, the signal processing circuit 603controls operation of the power receiving device antenna circuit 602.That is, the signal processing circuit 603 can control the intensity,the frequency, or the like of a signal transmitted by the powerreceiving device antenna circuit 602. The power load portion 610 is adriving portion which receives electric power from the lithium ionsecondary battery 604 and drives the power receiving device 600. Typicalexamples of the power load portion 610 include a motor, a drivingcircuit, and the like. Another device which drives the power receivingdevice by receiving electric power can be used as the power load portion610 as appropriate. The power feeding device antenna circuit 701 has afunction of transmitting a signal to the power receiving device antennacircuit 602 and receiving a signal from the power receiving deviceantenna circuit 602. The signal processing circuit 702 processes asignal received by the power feeding device antenna circuit 701. Inaddition, the signal processing circuit 702 controls operation of thepower feeding device antenna circuit 701. That is, the signal processingcircuit 702 can control the intensity, the frequency, or the like of asignal transmitted by the power feeding device antenna circuit 701.

The lithium ion secondary battery according to one embodiment of thepresent invention is used as the lithium ion secondary battery 604included in the power receiving device 600 in the RF power feedingsystem shown in FIG. 6.

With the use of the lithium ion secondary battery according to oneembodiment of the present invention for the RF power feeding system, thedischarge capacity or the charge capacity (also referred to as theamount of power storage) can be high as compared with that of aconventional lithium ion secondary battery. Therefore, since the timeinterval of the wireless power feeding can be longer, power feeding canbe less frequent.

In addition, with the use of the lithium ion secondary battery accordingto one embodiment of the present invention for the RF power feedingsystem, the power receiving device 600 can be compact and lightweight ifthe discharge capacity or the charge capacity with which the power loadportion 610 can be driven is the same as that of a conventionalsecondary battery. Therefore, the total cost can be reduced.

Next, another example of the RF power feeding system will be describedwith reference to FIG. 7.

In FIG. 7, the power receiving device 600 includes the power receivingdevice portion 601 and the power load portion 610. The power receivingdevice portion 601 includes at least the power receiving device antennacircuit 602, the signal processing circuit 603, the lithium ionsecondary battery 604, a rectifier circuit 605, a modulation circuit606, and a power supply circuit 607. In addition, the power feedingdevice 700 includes at least the power feeding device antenna circuit701, the signal processing circuit 702, a rectifier circuit 703, amodulation circuit 704, a demodulation circuit 705, and an oscillatorcircuit 706.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 and transmitting a signal to the power feeding deviceantenna circuit 701. In the case where the power receiving deviceantenna circuit 602 receives a signal transmitted by the power feedingdevice antenna circuit 701, the rectifier circuit 605 has a function ofgenerating DC voltage from the signal received by the power receivingdevice antenna circuit 602. The signal processing circuit 603 has afunction of processing a signal received by the power receiving deviceantenna circuit 602 and controlling charge of the lithium ion secondarybattery 604 and supply of electric power from the lithium ion secondarybattery 604 to the power supply circuit 607. The power supply circuit607 has a function of converting voltage stored in the lithium ionsecondary battery 604 into voltage needed for the power load portion610. The modulation circuit 606 is used when a certain response istransmitted from the power receiving device 600 to the power feedingdevice 700.

With the power supply circuit 607, electric power supplied to the powerload portion 610 can be controlled. Thus, overvoltage application to thepower load portion 610 can be inhibited, and deterioration or breakdownof the power receiving device 600 can be inhibited.

In addition, with the modulation circuit 606, a signal can betransmitted from the power receiving device 600 to the power feedingdevice 700. Therefore, when the amount of charged power in the powerreceiving device 600 is detected and a certain amount of power ischarged, a signal is transmitted from the power receiving device 600 tothe power feeding device 700 so that power feeding from the powerfeeding device 700 to the power receiving device 600 can be stopped. Asa result, the lithium ion secondary battery 604 is not fully charged, sothat the number of charge cycles of the lithium ion secondary battery604 can be increased.

The power feeding device antenna circuit 701 has a function oftransmitting a signal to the power receiving device antenna circuit 602and receiving a signal from the power receiving device antenna circuit602. When a signal is transmitted to the power receiving device antennacircuit 602, the signal processing circuit 702 generates a signal whichis transmitted to the power receiving device. The oscillator circuit 706is a circuit which generates a signal with a constant frequency. Themodulation circuit 704 has a function of applying voltage to the powerfeeding device antenna circuit 701 in accordance with the signalgenerated by the signal processing circuit 702 and the signal with aconstant frequency generated by the oscillator circuit 706. Thus, asignal is output from the power feeding device antenna circuit 701. Onthe other hand, when a signal is received from the power receivingdevice antenna circuit 602, the rectifier circuit 703 has a function ofrectifying the received signal. From signals rectified by the rectifiercircuit 703, the demodulation circuit 705 extracts a signal transmittedfrom the power receiving device 600 to the power feeding device 700. Thesignal processing circuit 702 has a function of analyzing the signalextracted by the demodulation circuit 705.

Note that any circuit may be provided between the circuits as long asthe RF power feeding can be performed. For example, after the powerreceiving device 600 receives a signal and the rectifier circuit 605generates DC voltage, a circuit such as a DC-DC converter or regulatorthat is provided in a subsequent stage may generate constant voltage.Thus, overvoltage application to the inside of the power receivingdevice 600 can be inhibited.

The lithium ion secondary battery according to one embodiment of thepresent invention is used as the lithium ion secondary battery 604included in the power receiving device 600 in the RF power feedingsystem shown in FIG. 7.

With the use of the lithium ion secondary battery according to oneembodiment of the present invention in the RF power feeding system, thedischarge capacity or the charge capacity can be increased as comparedwith that of a conventional lithium ion secondary battery; therefore,the time interval of the wireless power feeding can be longer, so thatpower feeding can be less frequent.

In addition, by using the lithium ion secondary battery according to oneembodiment of the present invention in the RF power feeding system, thepower receiving device 600 can be compact and lightweight if thedischarge capacity or the charge capacity with which the power loadportion 610 can be driven is the same as that of a conventional lithiumion secondary battery. Therefore, the total cost can be reduced.

Note that when the lithium ion secondary battery according to oneembodiment of the present invention is used in the RF power feedingsystem and the power receiving device antenna circuit 602 and thelithium ion secondary battery 604 overlap with each other, it ispreferred that the impedance of the power receiving device antennacircuit 602 is not changed by deformation of the lithium ion secondarybattery 604 due to charge and discharge of the lithium ion secondarybattery 604 and deformation of an antenna due to the above deformation.If the impedance of the antenna is changed, in some cases, electricpower is not supplied sufficiently. For example, the lithium ionsecondary battery 604 may be placed in a battery pack formed of metal orceramics. Note that in that case, the power receiving device antennacircuit 602 and the battery pack are preferably separated from eachother by several tens of micrometers or more.

In this embodiment, the signal for charge has no limitation on itsfrequency and may have any band of frequency with which electric powercan be transmitted. For example, the signal for charge may have any ofan LF band of 135 kHz (long wave), an HF band of 13.56 MHz (short wave),a UHF band of 900 MHz to 1 GHz (ultra high frequency wave), and amicrowave band of 2.45 GHz.

A signal transmission method may be properly selected from variousmethods including an electromagnetic coupling method, an electromagneticinduction method, a resonance method, and a microwave method. In orderto prevent energy loss due to foreign substances containing moisture,such as rain and mud, the electromagnetic induction method or theresonance method using a low frequency band, specifically, frequenciesof short waves of from 3 MHz to 30 MHz, frequencies of medium waves offrom 300 kHz to 3 MHz, frequencies of long waves of from 30 kHz to 300kHz, or frequencies of ultra long waves of from 3 kHz to 30 kHz, ispreferably used.

This embodiment can be implemented in combination with any of theabove-described embodiments.

This application is based on Japanese Patent Application serial No.2011-060197 filed with Japan Patent Office on Mar. 18, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A lithium ion secondary battery comprising: a positiveelectrode comprising: a positive electrode current collector; and apositive electrode active material layer over the positive electrodecurrent collector, the positive electrode active material layercomprising a plurality of single crystal particles, wherein each of thesingle crystal particles is a lithium-containing composite oxide havingan olivine structure, wherein in each of the single crystal particles, alength in a b-axis direction is shorter than each of lengths in ana-axis direction and a c-axis direction, and wherein a ratio of lengthsof the single crystal particles in the a-axis direction and the c-axisdirection is greater than or equal to 0.8 and less than or equal to 1.2.3. The lithium ion secondary battery according to claim 2, wherein eachlength in the b-axis direction of the single crystal particles isgreater than or equal to 5 nm and less than or equal to 50 nm.
 4. Thelithium ion secondary battery according to claim 2, wherein the b-axisof each of the single crystal particles intersects with a surface of thepositive electrode current collector at an angle greater than or equalto 60 degrees and less than or equal to 90 degrees.
 5. The lithium ionsecondary battery according to claim 2, wherein at least one of thesingle crystal particles is overlapped with at least another one of thesingle crystal particles.
 6. The lithium ion secondary battery accordingto claim 2, wherein the lithium-containing composite oxide is expressedby LiMPO₄ (M is one or more of Fe (II), Mn (II), Co (II), and Ni (II)).7. The lithium ion secondary battery according to claim 2, wherein eachof the single crystal particles is a flat single crystal particle. 8.The lithium ion secondary battery according to claim 2, wherein in eachof the single crystal particles, a length in a b-axis direction isshorter than each of lengths in the a-axis direction and the c-axisdirection, and a b-plane has a square shape or a substantially squareshape.
 9. A lithium ion secondary battery comprising: a positiveelectrode comprising: a positive electrode current collector; and apositive electrode active material layer over the positive electrodecurrent collector, the positive electrode active material layercomprising a plurality of single crystal particles, wherein each of thesingle crystal particles is a lithium-containing composite oxide havingan olivine structure, wherein in each of the single crystal particles, alength in a b-axis direction is shorter than each of lengths in ana-axis direction and a c-axis direction, wherein a ratio of lengths ofthe single crystal particles in the a-axis direction and the c-axisdirection is greater than or equal to 0.8 and less than or equal to 1.2,and wherein a b-axis of each of the single crystal particles intersectswith a surface of the positive electrode current collector.
 10. Thelithium ion secondary battery according to claim 9, wherein each lengthin the b-axis direction of the single crystal particles is greater thanor equal to 5 nm and less than or equal to 50 nm.
 11. The lithium ionsecondary battery according to claim 9, wherein the b-axis of each ofthe single crystal particles intersects with the surface of the positiveelectrode current collector at an angle greater than or equal to 60degrees and less than or equal to 90 degrees.
 12. The lithium ionsecondary battery according to claim 9, wherein at least one of thesingle crystal particles is overlapped with at least another one of thesingle crystal particles.
 13. The lithium ion secondary batteryaccording to claim 9, wherein the lithium-containing composite oxide isexpressed by LiMPO₄ (M is one or more of Fe (II), Mn (II), Co (II), andNi (II)).
 14. The lithium ion secondary battery according to claim 9,wherein each of the single crystal particles is a flat single crystalparticle.
 15. The lithium ion secondary battery according to claim 9,wherein in each of the single crystal particles, a length in a b-axisdirection is shorter than each of lengths in the a-axis direction andthe c-axis direction, and a b-plane has a square shape or asubstantially square shape.